FIELD OF THE INVENTION
This invention relates to the preparation of soluble medium molecular weight phosphazene polymers. More particularly, it relates to the preparation of medium molecular weight poly(aryloxyphosphazene) homopolymers and copolymers by the thermally-induced stabilization of high molecular weight homologs.
The preparation of poly(aryloxyphosphazene) homopolymers and copolymers has been disclosed in U.S. Pat. No. 3,732,175, Allcock; U.S. Pat. No. 3,856,712, Renard et al; U.S. Pat. No. 3,856,713, Rose et al; and U.S. Pat. No. 3,883,451, Renard et al. Allcock discloses cross linking of phosphazene polymers by ligand exchange so as to create higher molecular weight cross-linked materials. The copolymers described in the first-mentioned Renard et al patent contain selected quantities of both aryloxy and alkoxy side chains on the phosphazene backbone, and the copolymers described in the latter-mentioned Renard et al patent are characterized by the presence of halogen-substituted aryl side chains on the phosphazene backbone. The copolymers disclosed in the above-mentioned Rose et al patent are characterized by the presence of only aryloxy and alkyl-substituted aryloxy side chains. Other related prior art may be found in U.S. Pat. Nos. 3,515,688; 3,700,629; 3,702,833; and 3,856,712.
It is generally recognized that the prior art phosphazene polymers disclosed above are low-temperature, elastomeric and film-forming thermoplastics. Depending on the nature of the substituent on the phosphorus-nitrogen backbone, impressive hydrolytic and chemical stabilities have been found to be associated with these polymers. However, the prior art has uniformly associated phosphazene polymers with extreme ease of thermally-induced depolymerization. The tendency for poly(alkoxy and aryloxyphosphazenes) to undergo such degradation, forming oligemers, has been demonstrated, as has such tendency in the poly(fluoroalkoxy and bisaryloxy) systems. See, for example, Allcock, H. R. et al, Macromolecules, 7,284 (1976).
Recently, other phosphazene polymers have been disclosed, such containing substituted aryloxy-substituents (preferably substituted in the para position) on the phosphorus atoms in a non-regular fashion, which are represented by the following formulas: ##STR1## wherein R.sub.1 and R.sub.2 are the same or different and are hydrogen or C.sub.1 -C.sub.10 linear or branched alkyl or C.sub.1 -C.sub.4 linear or branched alkoxy substituted in any sterically permissible position on the phenoxy group. See U.S. patent application Ser. No. 661,862, filed Feb. 27, 1976. As is generally the case for polymeric organo phosphazenes, the above-disclosed polymers are castable and processable to a certain extent, such mainly being controlled by the solubility of these materials in the various solvents. However, polymerization techniques for the phosphazene polymers as a whole, and for the above alkoxy and aryloxy materials, result in uncontrollably high molecular weights, e.g. M.sub.w equals greater than 1 .times. 10.sup.6.
In general, the high molecular weight polyphosphazenes are disadvantageously soluble in solvents and process with extreme difficulty. Film casting is, therefore, strictly limited to relatively thin films of polyphosphazene. Processing can only practically incorporate small amounts of material, since common plastic processing techniques, e.g. rubber mills, Banbury mixers, and the like, require extensive amounts of processing time in order to achieve uniform mixing. Furthermore, in either solvent or processing techniques, temperatures developed during the process must be closely controlled since uncontrollable crosslinking, depolymerization or oligemer formation may start at temperatures as low as 100.degree. C.
There is, therefore, a need for phosphazene materials showing improved solubility in organic solvents, shortened mill processing times, and enhanced thermal stability at temperatures of greater than 100.degree. C.